Switching converter having a controllable load element

ABSTRACT

Switching converter to convert an input direct current (U E ) into at least one output direct current (U A ), wherein an internal auxiliary voltage (U H1 , U H2 ) is derived from at least one auxiliary winding (W H1 , W H2 ) of a transformer (Tr), a controllable load element (BAL) is provided for the output voltage, and a stop signal (s s ) can be fed to turn the output voltage of the switching converter ON/OFF, which is used to connect the load element (BAL) to the output direct current to such an extent that a short-circuit-like condition is produced, in which the output voltage (U a ) is adequately close to null, but the at least one auxiliary voltage (U H1 , U H2 ) is adequately high to supply the units associated therewith.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a National Phase Patent Application of InternationalApplication No. PCT/AT01/00302, filed on Sep. 24, 2001, which claimspriority of Austrian Patent Application Number A 1671/2000, filed Oct.2, 2000

The invention relates to a switching converter to convert an inputdirect current into at least one output direct current, wherein aprimary winding of a transformer may be connected via a controlledswitch to the input direct current, a control signal with a variablepulse duty factor is fed to the switch by a control circuit, andregulation is provided depending on secondary and/or primary currents orvoltages by changing at least the pulse duty factor of the controlsignal, an internal auxiliary voltage is derived from at least oneauxiliary winding, a controllable load element is provided for theoutput voltage, and a stop signal can be fed to turn the output voltageof the switching converter ON/OFF.

Switching converters of this type have become known in a large number ofembodiments, either as blocking converters or as flow converters. Thecontrol circuit usually includes a pulse width modulator, whose pulseduty factor may be changed for the purpose of regulation, in particularto a constant output voltage or a constant output current.

The turning ON/OFF of such a switching converter should often bepossible using a stop signal that can come, for example, from aconsuming device and should be at the secondary side potential level.Then, switching may be carried out using a switch of the primary side ifpotential separation is taken care of by an optical coupler or a relaydevice. The stop signal may, however, also be generated in the switchingconverter itself, e.g., upon occurrence of error conditions, such asshort circuits, etc.

Aside from the fact that this solution is expensive, all auxiliaryvoltages that supply the control and regulation circuits with currentare also turned off, a situation that is undesirable for variousreasons, for example, due to the time that is necessary until theconverter reaches a stationary stable state after being turned on again.In such cases, the auxiliary voltage(s) is/are obtained usingindependent additional converters permanently connected to the mainpower grid, or it is possible—if a potential separation of the auxiliaryvoltage from the main power grid is not necessary—to draw an auxiliaryvoltage directly from the main power grid, e.g., by means of alongitudinal regulator or a capacitive voltage divider.

One object of the invention is to enable de-energizing the converter onthe secondary side based on a stop signal without interrupting the powersupply of the converter-internal control and regulation units.

This object is accomplished with a switching converter of the typementioned in the introduction, wherein, according to the invention, thestop signal to connect the load element to the output direct current isused to such an extent that a short-circuit-like condition is produced,in which the output voltage is close enough to null, the at least oneauxiliary voltage is adequately high to supply the units associatedtherewith.

The invention thus uses the behavior of a switching converter that isregulated by pulse width modulation in that a short-circuit-likecondition is produced, whereby the pulse duty factor becomes small, butremains on an auxiliary winding for enough voltage that the associatedauxiliary voltage can supply the regulation/control units of theconverter. The output voltage remaining on the load element can be keptso slight that it is, de facto, null, for the consuming device.

An advantageous variant is distinguished by the fact that a loadregulator, to which a signal related to the output current is fed, isprovided for the control of the load element. Thus, the load element mayalso be used as a connected base load during idling. It can also beadvantageous if the load regulator is fed a signal related to thecurrent by the load element.

An additional improvement of the switching or regulation behavior can beachieved if the short-circuit-like condition of the control circuittriggered by the stop signal is fed the deviation between the auxiliaryvoltage and a reference voltage as a regulating signal.

In practice, it is expedient for the load element to be a controllablesemiconductor, in particular a transistor.

In many cases the circuit cost can be reduced if the load element can beconnected to the output voltage using a switch controlled by the stopsignal. It is particularly advantageous if the controlled switch is arelay. Then, the load element can also have a connectable resistor.

The invention along with additional advantages is explained in detail inthe following with reference to exemplary embodiments illustrated in thedrawings. They depict:

FIG. 1 a simplified schematic diagram of a switching converter accordingto the invention,

FIG. 2 a possible, simple implementation of a controllable load element,and

FIG. 3 a variant of a switching converter according to the invention,depicted in detail.

According to FIG. 1, a switching converter according to the inventionhas a transformer Tr with a primary winding W_(P), a secondary windingW_(S), as well as an auxiliary winding W_(H). The primary winding W_(P)can be connected via a controlled circuit S to an input direct currentU_(E), that is obtained, for example, by rectification of power gridvoltage. The circuit S is controlled by a control circuit AST thatdelivers a substantially square control signal s₀, via whose pulse dutyfactor regulation may occur, in particular of the output voltage and/orthe output current.

On the secondary side, the voltage of a secondary winding W_(S) isrectified using a diode D1 and fed via an inductor L to a capacitor C,to which the output direct current U_(a) is applied. In known fashion, afreewheeling diode D2 is provided. For the regulation of the outputvoltage U_(a), it—or a fraction thereof—is compared in a voltagecomparator K_(U) with a reference voltage U_(R1), and the output signalof the voltage comparator K_(U) is fed to the control circuit AST.Similarly, for the regulation or limiting of the output current I_(a),the signal of a current sensor S_(c), e.g., of a current converter or amultiplier, is compared in a current comparator K_(I) with a referencevoltage U_(R2), and the output signal of the current comparator K_(I) isalso fed to the control circuit AST.

In the exemplary embodiments depicted here, the switching converter isimplemented as a flow converter; it is, however, emphasized that theinvention described in further detail in the following is alsoapplicable in conjunction with blocking converters.

In parallel to the output voltage U_(a), a load element BAL, to which anexternal stop signal s_(s) can be fed, is provided. This will bediscussed in further detail below.

An auxiliary voltage U_(H) is obtained from an auxiliary winding W_(H)of the transformer Tr by rectification using a diode D_(H) andfiltration through a capacitor C_(H). More expensive and regulatedcircuits are, however, usually used to generate an auxiliary voltage.The auxiliary voltage U_(H), which is potentially separated both fromthe input voltage U_(E) and from the output voltage U_(a), can be usedto supply the regulation and control electronics of the switchingconverter or also for other purposes.

As already explained above, the converter or its output voltage U_(a)is, de facto, de-energized by the stop signal; however, the auxiliaryvoltage should be high enough to fulfill the purpose provided for it,e.g., all control and regulation of the converter during operation. Forthis purpose, the controllable load element BAL is used, which issupposed to generate, upon application of the stop signal, a high enoughload on the output of the switching converter that the output voltageU_(a) is, de facto, null, that the pulse duty factor provided by thecontrol circuit AST still remains high enough that the AC voltage on theauxiliary winding W_(H) and, thus, also the auxiliary DC voltage U_(H)remains high enough.

FIG. 2 depicts a very simple possible embodiment, according to which theload element BAL consists of a load resistor R_(B), that can be appliedvia a relay switch a to the output of the switching converter. The stopsignal S_(s) is, in this case, supplied to a relay coil A of the relay.In the presence of a stop signal and the corresponding closing of thecontact a, the switching converter is, de facto, changed to ashort-circuit condition, whereby the output current I_(a) is adjusted toa maximum value. For this, the output current I_(a) is measured by acurrent sensor S_(c) and compared in the current comparator K_(I) with areference voltage U_(R2), which specifies the maximum current. Theoutput signal of the comparator K_(I) is fed to the control circuit AST,which, in the now existing quasi short-circuit condition switches fromvoltage regulation via the comparator K_(U) to current regulation inknown fashion. This ensures that the output voltage is, in fact, verylow—depending on the magnitude of the load resistance R_(B), on theorder of a few hundred mV—however, a pulse duty factor that ensures anadequately high auxiliary voltage U_(H) is retained.

In the second embodiment of the invention depicted in greater detail inFIG. 3, the following details must also be mentioned in addition to thecircuit elements already enumerated in FIG. 1.

The input voltage U_(E) applied to an input capacitor C_(E) is fed tothe primary winding W_(P) via a controlled switch S, depicted here as atransistor; and the associated control circuit AST is supplied by anauxiliary voltage U_(H1), which is obtained by rectification of thevoltage applied to an auxiliary winding W_(H1) by a diode D_(H1) andfiltration through a capacitor C_(H1).

On the secondary side, the load element BAL is designed as a transistorwhich lies with its collector-emitter section parallel to the outputcapacitor C. For the control of the load element BAL, i.e., thetransistor, a load regulator BAR is provided, which is, for example,supplied by a second auxiliary voltage U_(H2). This auxiliary voltageU_(H2) is obtained by rectification of the voltage on an auxiliarywinding W_(H2) by a diode D_(H2) and subsequent filtration through acapacitor C_(H2).

As according to FIG. 1, here again, a voltage comparator K_(U) as wellas a current comparator K_(I) is provided, whereby the outputs of thesetwo comparators K_(U) and K_(I) are fed by decoupling diodes D_(U) andD_(I) via a changeover switch S_(U) and an optical coupler OPO to thecontrol circuit AST.

Provision is made in this variant that upon appearance of the stopsignal s_(s), adjustment to the auxiliary voltage U_(H2) occurs. Inaddition, the current is measured by the transistor of the load elementBAL with a current sensor F_(c) and fed to the load regulator BAR, suchthat the current is also taken into account by the load element.

For regulation to the auxiliary voltage U_(H2), an additional comparatorK_(H) is provided, to one of whose inputs the auxiliary voltage U_(H2)or a fraction thereof is supplied, whereas a reference voltage U_(ref)is supplied to the other input of this comparator—just as to the otherrespective inputs of the comparators K_(U) and K_(I).

Using the changeover switch S_(U) controlled by the stop signal S_(s),the regulation of the voltage or current via the comparators K_(U) andK_(I) can be switched to the regulation to the auxiliary voltage U_(H2)via the comparator K_(H).

When, with this circuit, a stop signal S_(s) is supplied, for example,from a remote consuming device, the entire regulation switches via thechangeover switch S_(U) to the regulation of the auxiliary voltageU_(H2)—and, of course, also of U_(H1) because of the magnetic coupling.A current flows via the transistor of the load element BAL, by which thecurrent sensor F_(E) is limited using the load regulator BAR. Inaddition, via a signal S_(i) from the current sensor S_(c), afreewheeling state can be detected, whereby the load element BAL is thenconnected via the load regulator BAR to a base load.

In this variant of the invention as well, it is essential that ashort-circuit-like condition be introduced via the controllable loadelement BAL as soon as a stop signal appears. The load element, in thiscase, the transistor on the output, is set such that the output voltageis adequately low (de facto, null, for the respective consuming device),and that, on the other hand, the current through the load element isonly just large enough that the auxiliary supply voltages are adequatelyhigh.

It should be pointed out that combinational circuit parts often havemore than one output voltage, i.e., two or more decoupled DC outputvoltages, of which at least one is regulated. The load element in thecontext of the invention may then lie on one of the output voltagessince the other output voltages are coupled by the magnetic circuit.

1. A switching converter to convert an input direct current into atleast one output direct current comprising: a controlled switch; aprimary winding to accept via the controlled switch the input directcurrent; a control circuit for feeding a control signal with a variablepulse duty factor to the controlled switch and for regulating theswitching converter depending on a secondary current or voltage, or aprimary current or voltage, by changing at least the pulse duty factorof the control signal; at least one auxiliary winding from which aninternal auxiliary voltage is derived; and a controllable load elementcontrolled by a stop signal for electrically coupling and decoupling thecontrollable load element to and from an output voltage, wherein whenelectrically coupled to the output voltage, the controllable loadelement maintains the pulse duty factor of the control signal sufficientfor generating the internal auxiliary voltage required for operating theswitching converter.
 2. The switching converter of claim 1, furthercomprising a load regulator, to which a signal related to the outputcurrent is supplied, for controlling the controllable load element. 3.The switching converter of claim 2, wherein a signal related to acurrent through the controllable load element is supplied to the loadregulator.
 4. The switching converter of claims 1 or 2 or 3, wherein thedeviation between the auxiliary voltage and a reference voltage is fedas a control signal in the short-circuit-like condition of the controlcircuit triggered by the stop signal.
 5. The switching converter ofclaim 1, wherein the controllable load element is a controllablesemiconductor.
 6. The switching converter of claim 5, wherein thecontrollable load element is a transistor.
 7. The switching converter ofclaim 1, wherein the controllable load element can be connected to theoutput voltage using a switch controlled by the stop signal.
 8. Theswitching converter of claim 7, wherein the controlled switch is arelay.
 9. The switching converter of claims 7 or 8, wherein thecontrollable load element has a connectable resistor.
 10. A switchingconverter for converting an input direct current into an output directcurrent, comprising: a controlled switch controlled by varying a pulseduty factor of a control signal; a primary winding coupled via thecontrolled switch to the input direct current; a control circuit forfeeding the control signal having the variable pulse duty factor to thecontrolled switch; an auxiliary winding for producing an internalauxiliary voltage for operating the switching converter; a secondarywinding for producing the output direct current and an output voltage;and a controllable load element electrically coupled to the outputvoltage by a stop signal for lowering the output voltage whilemaintaining the internal auxiliary voltage sufficient for operating theswitching converter.